Modern refineries employ many upgrading unit processes such as fluidic catalytic cracking (FCC), hydrocracking (HCR), alkylation, and paraffin isomerization. As a result, these refineries produce a significant amount of isopentane. Historically, isopentane was a desirable blending component for gasoline having a high octane rating (92 RON), although it exhibited high volatility (20.4 Reid vapor pressure (RVP)). As environmental laws began to place more stringent restrictions on gasoline volatility, the use of isopentane in gasoline was limited because of its high volatility. As a consequence, the problem of finding uses for by-product isopentane became serious, especially during the hot summer season. Moreover, as more gasoline compositions contain ethanol instead of MTBE as their oxygenate component, more isopentane had to be kept out of the gasoline pool in order to meet the gasoline volatility specification. Thus, gasoline volatility became an even more serious problem and limited the usefulness of isopentane as a gasoline blending component.
A novel alkylation process, which is disclosed in U.S. Patent Application Publication 2006/0131209 (“the '209 publication”), was developed whereby undesirable, excess isopentane is converted into desirable and much more valuable low-RVP gasoline blending components. The contents of the '209 publication incorporated by reference herein in their entirety. This alkylation process involves contacting isoparaffins, preferably isopentane, with olefins, preferably ethylene, in the presence of an ionic liquid catalyst to produce the low-RVP gasoline blending components. This process eliminates the need to store or otherwise use by-product isopentane and eliminates concerns associated with such storage and usage. Furthermore, the ionic liquid catalyst in this process can also be used with conventional alkylation feed components (e.g. isobutane, propylene, butene, and pentene).
The ionic liquid catalyst distinguishes this novel alkylation process from conventional processes that convert light paraffins and light olefins to more lucrative products such as the alkylation of isoparaffins with olefins and the polymerization of olefins. For example, one of the most extensively used processes is the alkylation of isobutane with C3-C5 olefins to make gasoline cuts with high octane numbers. However, all conventional alkylation processes employ sulfuric acid (H2SO4) and hydrofluoric acid (HF) catalysts.
Numerous disadvantages are associated with the use of H2SO4 and HF catalysts. Extremely large amounts of acid are necessary to initially fill the reactor. A H2SO4 plant also requires a huge amount of daily withdrawal of spent acid for off-site regeneration, which involves incinerating the spent H2SO4 to recover SO2/SO3 and preparing fresh H2SO4. While an HF alkylation plant has on-site regeneration capability and daily make-up of HF is orders of magnitude less, HF forms aerosol. Aerosol formation presents a potentially significant environmental risk and also makes the HF alkylation more dangerous than the H2SO4 alkylation. This is evident from additional safety measures associated with modern HF alkylation processes such as water spray and catalyst additive for aerosol reduction. The ionic liquid catalyst alkylation process overcomes such disadvantages and fulfills the apparent need for safer and more environmentally-friendly catalyst systems.
Benefits of the ionic liquid catalyst alkylation process include the following:
(1) significant environmental, health and safety advantages;
(2) substantial reduction in capital expenditure as compared to H2SO4 and HF alkylation plants;
(3) substantial reduction in operating expenditures as compared to H2SO4 alkylation plants;
(4) substantial reduction in catalyst inventory volume (potentially by 90%);
(5) substantial reduction in catalyst make-up rate (potentially by 98% compared to H2SO4 plants);
(6) higher gasoline yield;
(7) comparable or better product quality (Octane number, RVP, T50);
(8) expansion of alkylation feeds to include isopentane and ethylene; and
(9) higher activity and selectivity of the catalyst.
Ionic liquid catalysts specifically useful in the alkylation process described in the '209 publication are disclosed in U.S. Patent Application Publication 2006/0135839 (“the '839 publication”), which is also incorporated by reference in its entirety herein. Such catalysts include a chloroaluminate ionic liquid catalyst comprising a hydrocarbyl substituted pyridinium halide of the general formula A below and aluminum trichloride or a hydrocarbyl substituted imidazolium halide of the general formula B below and aluminum trichloride. To prepare this chloroaluminate ionic liquid catalyst, 1 molar equivalent hydrocarbyl substituted pyridinium halide or hydrocarbyl substituted imidazolium halide can be combined with 2 molar equivalents aluminum trichloride. Such catalysts further include a chloroaluminate ionic liquid catalyst comprising an alkyl substituted pyridinium halide of the general formula A below and aluminum trichloride or an alkyl substituted imidazolium halide of the general formula B below and aluminum trichloride. To prepare this chloroaluminate ionic liquid catalyst, 1 molar equivalent alkyl substituted pyridinium halide or alkyl substituted imidazolium halide can be combined with 2 molar equivalents aluminum trichloride.
wherein R═H, methyl, ethyl, propyl, butyl, pentyl or hexyl group and X is a haloaluminate and preferably a chloroaluminate, and R1 and R2═H, methyl, ethyl, propyl, butyl, pentyl, or hexyl group and where R1 and R2 may or may not be the same. Preferred chloroaluminate ionic liquid catalysts include 1-butyl-4-methyl-pyridinium chloroaluminate (BMP), 1-butyl-pyridinium chloroaluminate (BP), 1-butyl-3-methyl-imidazolium chloroaluminate (BMIM) and 1-H-pyridinium chloroaluminate (HP).
However, ionic liquid catalysts have unique properties making it necessary to further develop and modify the ionic liquid catalyzed alkylation process in order to achieve superior gasoline blending component products, improved process operability and reliability, reduced operating costs, etc. One of the unique properties of an ionic liquid catalyst is its much higher activity in catalyzing alkylation reactions than conventional sulfuric acid and hydrofluoric acid catalysts.
In conventional alkylation processes, due to relatively low catalyst activity, a large amount of acid catalyst has to be used in the system, for example, 50-60 vol %. As a result, the acid catalyst forms a continuous phase in the alkylation reactor while the hydrocarbon reactants (i.e., isoparaffin and olefin) form a dispersed phase or small droplets suspended in the acid phase. In this liquid-liquid dispersion, a large interfacial area between the catalyst continuous phase and the hydrocarbon dispersed phase can be achieved by conventional emulsifying techniques, such as high speed stirring and static mixing.
In contrast, in the ionic liquid alkylation process, a much smaller amount of ionic liquid catalyst is needed to catalyze the reactions with high selectivity. Usually, a 5-10 vol % of ionic liquid catalyst is sufficient to catalyze the reactions between isoparaffin and olefin. Under such conditions, the hydrocarbon phase forms a continuous phase while the ionic liquid forms a dispersed phase or small droplets suspended in the hydrocarbon phase. This liquid-liquid dispersion requires highly intimate contact between the catalyst and the hydrocarbon phases. Due to the low inventory of ionic liquid catalyst in the reactor system, a large interfacial area between the two liquid phases cannot be achieved by conventional emulsifying techniques. Due to the large density difference between the ionic liquid catalyst and hydrocarbon phase, ionic liquid droplets, if not small enough, will quickly settle down and be segregated from the hydrocarbon phase, resulting in a very short contact time between the phases that is not sufficient to catalyze the alkylation reaction. Finally, due to the relatively large vol % of hydrocarbon phase in the reactor, good mixing is required to homogenize the hydrocarbon phase and achieve a uniform composition of the hydrocarbon phase and a uniform temperature gradient throughout the reactor.
Thus, the ionic liquid catalyst alkylation process requires intimate mixing of the hydrocarbons and catalyst, sufficient interfacial contact between the hydrocarbons and catalyst, minimal residence time distribution, good temperature and pressure control, and a high isoparaffin to olefin (I/O) ratio. The ionic liquid alkylation process is also a highly exothermic reaction necessitating prompt heat removal.
WO Patent No. 98/31454 discloses a reactor system using a static mixer to emulsify sulfuric acid and hydrocarbon feeds in a sulfuric acid alkylation process. It also discloses several sulfuric acid alkylation processes using a high speed stirrer to emulsify feeds. However, it is well known that a high speed stirrer is not energy efficient in emulsifying a liquid-liquid system. This is especially true for the ionic liquid catalyzed alkylation process, where a small amount of much heavier and more viscous ionic liquid phase must be emulsified in a light hydrocarbon phase. As for the static mixer, it is also well known that a very high linear liquid velocity is required to emulsify liquid-liquid systems resulting in an prohibitively high pressure drop across the static mixer.
Above all, the ionic liquid catalyzed alkylation process is a unique process as compared to other conventional alkylation processes. There remains a need, however, to emulsify the ionic liquid and the hydrocarbon phase to achieve intimate contact between the phases for greater alkylation product quality and reaction control.
U.S. Pat. No. 3,696,168 (“Vanderveen”) discloses the use of several nozzles assembled into a compound nozzle system in an alkylation process using HF catalyst to achieve an increase in octane number and a related reduction in overall reaction temperature in addition to a reduction in temperature at immediate or exact point of contact between the hydrocarbons and the catalyst. A series of adjacently disposed nozzles spray fresh catalyst-free isoparaffin, fresh catalyst-free olefin, and recycled isoparaffin containing catalyst into a main body of catalyst where the alkylation reaction occurs. More particularly, a first conduit containing fresh isoparaffin, a second conduit containing fresh olefin, and a third conduit containing recycled isoparaffin are split into a series of conduits that carry a portion of each of the fresh isoparaffin, the fresh olefin, and the recycled isoparaffin to the nozzles. Each nozzle is constructed such that the isoparaffin and olefin pass through a centrally or axially disposed feed nozzle and the recycled isobutane passes through at least one subordinate or auxiliary feed nozzle next to the centrally disposed feed nozzle, wherein the feed nozzles are located within a cap or mantle.
While the teachings of Vanderveen impart certain benefits to the HF catalyzed alkylation process, there still exists a need for an improved alkylation process for converting isoparaffins and olefins in the presence of an ionic liquid catalyst.